Let’s talk about the history of sharks.
Did you know that the earliest references to sharks go as far back as the Silurian period some 450 million years ago? That’s millions of years before the first dinosaurs. Did you also know that many prehistoric sharks, like the helicoprion, evolved some astonishing features to enable them hunt and feed better?
Most people still assume that sharks have the same kind of streamlined shape, prominent dorsal fins, and terrifying pointy teeth. But that’s far from the truth. For instance, the tasselled wobbegong shark has a broad, flat shape and looks like an old carpet. Another species, the frilled shark looks nothing like the torpedo-shape of the great white, but its array of teeth are just as intimidating. It looks more like an eel than a shark but boasts averagely 300 numbers of trident-shaped teeth arranged in 25 rows!
What about species like the cookie-cutter, the sawfish, the basking shark, the goblin shark, or even the ninja lanternshark that glows in the dark? You can see that sharks are so varied, we simply can’t categorize them in basic terms. If that sounds awesome, consider this; fossil records indicate that there were up to 3000 different shark species in the oceans at some point. Simply amazing.
Though new sharks are still being discovered, we know of about 500 species that exist to date.
The Early Ancestors Of Modern Sharks
Scales found in Siberian deposits show that sharks first appeared about 450 million years ago in the late Silurian period. Climatic conditions on the Earth were warm and stable, sea levels were high, and coral reefs were just forming.
As other fish evolved, a group of fish called acanthodians (spiny sharks) developed. These fish are now extinct, but they looked like small sharks with different numbers of fins. They shared features with both bony fish and cartilaginous fish, and from all indications, sharks came from this group of fish.
The first verified shark teeth fossils date 50 million years after the Silurian period. They belonged to the Leonodus shark, a shark that swam during the early Devonian period. The teeth were tiny, just 4mm, and two-pronged but closely resembled that of Xenacanthus, another shark that appeared millions of years later.
For some reason, fish diversified greatly during the Devonian period. As a result, that period is named the “Age of Fishes.” For example, the skeleton of the extinct Cladoselache shows how much it differed from the eel-like sharks before it. The Cladoselache shark was about 2 meters long with large eyes and a torpedo shape. It must have been a fast swimmer because it appears it took its prey tail-first.
At this same time, another intimidating group of fish swam in the seas. These fish were called Dunkleosteus, and each one was about the size of a school bus. Because these fish were heavily-armored giants, they may have competed with sharks for prey. Maybe their presence was the catalyst that made the early sharks evolve so much to enable them out-live these formidable competitors.
A Period Of Intense Speciation In The History Of Sharks
The next clue to shark evolution can be traced to 380 million years ago through the fossilized braincase of a shark from Antarctica. Researchers deduced that it was an eel-like shark based on its head, teeth, fin, and spine. That shark is named Antarctilamna.
20 million years later during the Carboniferous period, sharks were evolving so rapidly that it would be almost impossible to describe each species. Here are just a few examples:
– Cartilage fish (Chondrichthyans). These were the largest predators in the oceans at that time. Such fish included skates, rays, and chimaeras. Chimaeras are known informally as a group including ghost sharks, ratfish, rabbitfish, and spookfish. According to Dr. Charlie Underwood a lecturer in Palaeontology at Birbeck University, the more weird and wonderful sharks evolved from the chimaera branch. You can read more about his research on sharks here.
– The Helicoprion was a shark that developed a very unique dental feature. It had a tooth whorl. A kind of dinner-plate sized spiral-shaped tooth structure. Each tooth whorl could be up to 40cm across and looked somewhat like a buzzsaw attached to its lower jaw. The old teeth of other shark species fall off to be replaced by new teeth but not the Helicoprion’s. Rather than falling off, the teeth just stick together at the lower jaw in a kind of 360-degree spiral arrangement.
– Another interesting species was the Stethacanthus sharks. The male of the species had a prominent anvil-shaped dorsal fin. No one knows for sure what this fin was used for.
– The Jurassic period saw the emergence of 12 new groups of sharks with flexible jaws. Christopher Bird of the National Oceanography Centre (NOC) at Southampton, says that this feature allowed them to feed on larger prey, even larger than themselves.
How Did Sharks Survive The Mass Extinction Of The Cretaceous Period?
The Cretaceous period was one of the most notorious climatic times ever in the history of Earth. At the start of this period, dinosaurs flourished, flowering plants were appearing on land, and land mammals were abundant. The breakup of Pangaea, the supercontinent, was underway. Temperatures were warm and stable, and sea levels were high. But, by the end of this period, the climate had changed severely. Consequently, there was high volcanic activity and lowered sea levels. Dinosaurs went out with a bang, and so did a staggering number of prehistoric life forms on land and in the seas.
However, sharks lived on.
Obviously, their ability to adapt quickly to the changing environment helped. After the exit of dinosaurs, there was an emergence of a second wave of deep-sea sharks. Species like the cookie-cutter and the lantern sharks became prominent after this post-crisis event according to Mr. Bird.
They were able to explore deeper habitats, and they even survived periods when the ocean lost oxygen. A situation that caused the death of many other larger species. Sharks sought refuge by moving deeper underwater and some developed bioluminescence.
Another interesting adaption is that of the Epaulette shark. This shark can survive for hours on little oxygen and can walk on land for short periods of time. It’s most likely these properties were triggered by dwindling sea levels.
The Rise And Fall Of The Megalodon Shark
Though sharks flourished by the end of the Cretaceous period, some of the larger species did not linger around for long. Most noticeable among them is the megalodon shark.
The megalodon measured between 13 and 17 meters (43 – 56 feet) in length and some fossils measured close to 25 meters (82 feet). It had teeth that measured up to 17.8 cm (7 inches) and were so large, people thought the fossilized teeth were dragon tongues for many years. In addition, the megalodon’s mouth could open up to 2 meters across. No doubt, this was the biggest prehistoric shark that ever existed. Also, it remains the largest marine predator in the history of this planet to date and easily dwarfs the modern Great White Shark many times over.
No one knows why it went extinct, but the most likely reason must have been a lack of sufficiently sized prey to feed its huge appetite.
What Can We Learn From The History Of Sharks?
From all the above information, we can see that sharks are tenacious and they have been able to thrive in a changing world for over 450 million years.
Sharks have evolved again and again to survive major upheavals in climatic conditions and the demise of many other animals, like the dinosaurs. But, they may not be able to evolve fast enough or reproduce quickly enough to survive what may well be the greatest threat to shark existence ever. Human activity.
Aside from unfavorable climatic changes, will human activity like habitat degradation, overfishing and pollution finally be the end of these remarkable creatures? That would be such a shame.
For now, they are still surviving with more new species still being discovered in recent times. Some of the recent discoveries within the last century include the hammerhead sharks, the ninja lanternsharks, and the megamouth shark. Others are the walking bamboo shark, the spotted belly catshark, and the Pacific nurse shark, etc.